Apparatus for interrupting electric circuits



Aug. 11, 1942. E.- K. SADLER 2,292,547

APPARATUS 'FOR INTERRUPTING ELECTRIC CIRCUITS Filed Sept. 13, 1939 2 Sheets-Sheet 1 5e Ikql.

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APPARATUS FOR INTERRUPTING ELECTRIC CIRCUITS Filed Sept. 13,- 1939 2 Sheets-Sheet 2 /Nl/E/V TOR ER/VEJ 7' K JADLER Q3 Y HARP/d, K/ECH, FOGTEI? a, HA RR/J FOR THE FIRM A Tromwsms Patented Aug. 11, 1942 APPARATUS FOR. INTERRUPTING ELECTRIC CIRCUITS Ernest K. Sadler, Glendale, CaliL, assignor to Kelman Electric & Manufacturing Company, Los Angeles, Calif., a corporation of California Application September 13, 1939, Serial No. 294,643 (Cl. 200-150) Claims. My invention relates to circuit interrupters and, more particularly, to electric circuit break-' ers of the type which interrupts a high-voltage circuit by the formation and quenching of an arc in a dielectric fluid.

It has previously been proposed to force a blast of oil or other dielectric fluid through the space between two rapidly separating contact members connected in an electric circuit to quench an arc therebetween. One of the simplest and most effective ways of creating such an arc-quenching blast is by generation of pressure through use of another are, usually connected in series with the arc to be quenched. For example, the pressure surge may be established by an explosive are between a generating set of contacts, this pressure surge being used to force a stream of dielectric fluid transversely across a set of interrupting contacts.

With certain devices of this general character, two objections have sometimes been apparent. In the first place, diificulty has sometimes been encountered in maintaining the flow of arc-extinguishing fluid for a suflicient time to interrupt the circuit. In the second place, certain low-current arcs, for example, those formed when interrupting light loads or merely the charging current 9f 9. transmission line, sometimes fail to give an explosive surge of such vio- Ience as to quench quickly the are between the interrupting set of contacts. In the latter connection, some circuit breakers of this type encounter diificulty in interrupting currents of less than 100-200 amperes even though they operate very satisfactorily at higher currents. I It is an object of the present invention to provide a simple current interrupter effective to interruptcurrents of various magnitudes.

Another object of the invention is to deliver a dielectric fluid to an are using both physicallyacting and electrically-acting means to force the arc-quenching fluid into the arc. For example, by the present invention I can combine the desirable generation of pressure surges by an electric arc with generation of pressure by use of a pump or other energy-storing means.

Another object of the invention is to provide a novel method and apparatus for circuit interruption which maintains a flow of dielectric fluid 'into a gap between a pair of separating contact members until the arc is eiiectively quenched, even though the arc might be sustained for several cycles.

An are formed between such separating contact members is surprisingly small in cross-sectional area, being about 1 sq. mm. per 1000 amperes, even though the corona-like glow surrounding same is of considerably larger area. On the other hand, it is necessary for various reasons to use contact members having av crossseotional area very considerably larger, for example, 1 square inch or more. Considerable difliculty is encountered in insuring a suflicient arcquenching flow when the gap between such contact members is relatively. small in length, for example, A; inch to inch.

It is an object of this invention to provide an eifective arc-quenching flow of dielectric fluid between a pair of separating contact members even when they are separated only slightly.

It is a further object of the invention to start to. lave the contact members with a dielectric fluid before they separate to form the arc.

Another object is to provide a pump means in which energy can be stored by the closing movement of the switch, wherebythis energy can be released to produce a flow of dielectric fluid when the circuit breaker is tripped.

Still further objects of the invention lie in the structural details, the position and the point of discharge of the pump means, and in the other features of the invention which will be evident to those skilled in the art from the following description of selected embodiments of the invention.

Referring to the drawings,

Fig.1 is a vertical cross-sectional view of an arc-quenching unit illustrating one embodiment of the invention, the contact members being in closed-switch position.

2 is a fragmentary cross-sectional view similar to Fig. 1, showing the contact members in a position assumed during the circuit-interrupting movement.

Fig. 3 is a sectional view similar to Fig. 2, showing the contact members in a later-assumed position during the current-interruption cycle.

Fig. 4 is an end view of the arc-quenching unit, taken as indicated by the line lli of Fig. 2.

Fig. 5 is a sectional view taken as indicated by the line 55 of Fig. 1.

Fig. 6 is a vertical cross-sectional view similar to Fig. 1 except showing the contact members in a position assumed during the circuit-interrupting movement, illustrating an alternative embodiment of the invention.

Fig. 7 is a vertical cross-sectional view of a complete circuit breaker embodying the invention.

Fig. 8 is an end view of some of the mecha= nism of Fig. 7, taken on the line 3-3 of Fig. 7.

Fig. 9 is a cross-sectional view taken along the line 3-3 of Fig. 7.

Referring particularly to Fig. l, I have shown an arc-quenching unit. indicated generally by the numeral l3, as including an open-ended and relatively thick tube ll preferably formed of insulating material, such as fiber, Bakelite, or various ceramic materials. This tube It can be or any suitable shape but is shown as being circular in cross section. Generally speaking, it comprises a main passage l2 (see Fig. 4), which is preferably bellied in such manner as to form numerous longitudinal passages l3 (Fig. 1), the entire arc-quenching uni being prefer ly 811 merged in a dielectric fluid, which, for example, may be a high resistance oil.

To form the longitudinal passages I3 I prefer to utilize a plurality of bafiles, |3 held in spaced relationship by spacers l3, these bames and spacers being preferably of such length as to extend throughout the length of the tube II, the

baiiles being of such width as to traverse the main passage l2, as best shown in Fig. 4. Each baliie I3 is substantially imperforate except for openings l3 (Fig. 5) adapted to receive and being only slightly larger than the contact members to be hereinafter described. These openings ll of the various baffles are in alignment with each other to form operating zones through which the contact members may move.

In the embodiment shown in Fig, 1, three sets of contact members are disclosed, the end sets being indicated by the numerals 23 and 24 and being hereinafter termed interrupting sets of contact members, and the intermediate set bein indicated by the numeral 25 and comprising a generating set of contact members. The set 23 comprises essentially first and second contact members 23 and 23 to b hereinafter described in greater detail, while the set 24 comprises first and second contact members 33 and 3|. Similarly, the set 25 comprises first and second contact members 32 and 33. The contact members 23, 3|, and 33 are adapted to be moved in the operating zones defined by the openings iii.

The longitudinal passages l3 are traversed by the contact members 29, 3|, and 33, and these longitudinal passages are preferably formed by the spacers I6 in the manner best shown in Fig. 5. As there shown, each spacer comprises complementary elements 33 and 33 separated from each other to define one Of the longitudinal passages |3 which is bounded by the walls of the elements 35 and 36 and the surfaces of the adjacent bailles I5. The elements 33 and 33 are arcuately enlarged in the zone of the contact member 33 to form a chamber 31 extending around this contact member. A more restricted zone 33 communicates between the chamber 31 and an enlarged chamber 33, while a similar restricted zone 43 extends from the enlarged chamber 33 to the vicinity of the contact member 23.

A. throat 4| and an exit portion 42 complete the passage l3, the portion 42 being in open communication with the surrounding dielectric fluid. The complementary elements 33 and 33 are similarly formed in the zone between th contact members 3| and 33 so as to be symmetrical about the latter. Correspondingly, each longitudinal passage l3 in effect traverses the operating zone occupied by each of the contact members 23, 3|, and 33. It will be understood that a similar longitudinal passage I3 is formed between each of the bames I! even though the distance across some portions of the longitudinal passages II will be less toward the upper and lower portions of the arc-quenching unit due to the cylindrical configuration of the tube Also, it should be noted that above and below the baiiies l3 and spacers .l3 are disposed upper and lower filler elements 43 and 43. I have found it preferable to cut away a portion of, the upper filler element 43 to form an enlarged chamber 41 on each side of the generating set of contacts 23. This enlargement is particularly useful in the uppermost longitudinal passage and serves as a storage for dielectric fiuid. It will'beunderstood that this uppermost longitudinal passage, as well as the corresponding lowermost longit-udinal passage, is defined by suitable spacers By means to be hereinafterdiescribed in detail, the contact members 23, 3|, and 33 are adapted to be substantially simultaneously withdrawn from the operating zones defined by the openings |3. When the circuit breaker is closed, these contact members are in the position shown in Fig. 1, and in moving toward open-switch position they assume the intermediate positions indicated in Figs. 2 and 3. The maximum retraction of these contact members is indicated by the dotted lines 43 of Fig. 3, in which position they are substantially clear of the longitudinal passages |3. In the event that complete withdrawal is desired, tapered portions 33 of the tube II will guide these contact members into the proper zones during the first portion of the switch-closing movement,

It is usually desirable to connect the three sets of contact members in series with the circuit to be interrupted. In Fig. 1 the first contact member 23 may be connected to the circuit to be interrupted through a conductor 32, in which event the current will flow downward through the set of contact members 23 and be conducted to the set 23 through a conductor 33, moving upward through the set 23 and through a conductor 34 to the set 24, and returning to the circuit to be interrupted. Suitable electrostatic shields 33 may be disposed at the lower endof each of the contact members 23. 3|, and 33, and likewise at the upper ends of the contact members 23, 33, and 32 if desired, for equalizhtion of stress and prevention of arcing or corona discharge from sharp edges.

In the preferred embodiment of the invention. both contact members of each set are movable. By way of example, it is desired that the contact member 23 should follow the contact member 23 until the engaging surfaces reach the first longitudinal passage l3, after which further movement of the contact member 23 is resisted so that further retraction of the contact member 33 will form a gap between these members and establish an arc. It is desired to interrupt this are by flowing transversely into the gap at high velocity a stream of dielectric fluid. The present invention contemplates the use of both electrically and mechanically-induced pressure surges to set up such a blast of dielectric fiuid. The electrically-induced pressure surge is developed by the generating set of contact members 23, and when an arc forms therebetween, a violent pressure surge is developed which forces outward along the upper longitudinal passage streams of dielectric fiuid moving respectively toward the contact members 23 and 3| and into the gaps thereadiacent. However, this electrically-induced pressure surge is usually of short duexcept for escape through the means to be presently described. On the other hand, before sealing engagement between the ledge 18 and the wall 88 takes place, the circuit breaker may be so designed as to produce a small flow'of dielectxic fluid from the pressure zone 68 through the opening 61 to lave the contact members prior to the time they separate.

The pressure in the pressure zone 88, when the elements are in the position shown in Fig. 2, is used to deliver a stream of dielectric fluid to a point adjacent the gap and. in the embodiment shown, into the first longitudinal passage II. This is accomplished by a passage communicating with the pressure zone 88 and discharging dielectric fluid adjacent the gap. In the embodiment of Fig. i this passage is indicated by the numeral 88 and is formed by intersecting longitudinal and transverse bores in the contact member 28. In this embodiment, it will be clear that the engaging surfaces of the first and second contact members 28 and 28 forman effective 'seal preventing flow of dielectric fluid through the passage 88 until such time as the second contact member 29 separates from the flrst contact tacts 24 and 28 act in a similar manner. In addition, however, the pump means associated with the set 25 cooperates novelly with the interrupting sets 23 and 24 in that it delivers a stream of dielectric fluid to the are between this generating set of contact members, and continues this delivery to force dielectric fluid outward through the passages in accumulating sequence, thus insuring that the gaps between the interrupting sets of contact members 28 and 24 will be fllled with transverse jets of dielectric fluid regardless of the length of these gaps. In addition, it will be apparent that the contact member 88 of the generating set 25 acts as a valve to open the different passages l8 successively and in so doing draws the are into the vicinity of the dielectric fluid retained in the several chambers 81, thus further increasing the dielectric fluid available for vaporization and generation of pressure acting to force dielectric fluid along the several passages i8 toward the interrupting sets 28 and 24.

It will be apparent that the flow of dielectric fluid from the pressure zone 88 will continue for a time determined by numerous factors including the strength of the spring 14 and the throttling action of the passage 88. It is preferred to continue this flow into the several gaps until the contact members 29, 8|, and 88 have assumed their maximum-retracted position which, in the embodiment shown, is represented by the dotted lines 49 of Fig. 3. The invention thus insures an adequate delivery of dielectric fluid to the gaps even though, as is sometimes the case in interrupting currents of relatively low magnitude, the arc continues for several cycles if an alternating current circuit is being interrupted.

It is preferred to open the generating set of contact members 25 slightly ahead of the time that the interrupting sets 28 and 24 open. This condition is shown in Fig. 2 and insures an electrically-induced flow of dielectric fluid across the interrupting sets 28 and 24 at the instant of separation. On the other hand, it is sometimes difiicult to force a stream of dielectric fluid through a gap as small as V, inch between the contact members 28 and 28, particularly if the diameter of these contacts is considerably larger. The pump means 88 associated with these contact members will, however, forcibly move dielectric fluid into the gap even at this small spacing of the contact members, and as soon as the gap lengthens, the transverse flow induced by the generating set is available for cooling and sweeping the are from the gap between the interrupting set.

An alternative embodiment of the invention is shown in Fig. 6, which resembles, in structural details, the form shown in Fig. 1 except that a different form of passage 88 is shown and a different means is .used for stopping the motion of the flrst contact member 82 of the generating set 28. In this embodiment the contact member 82 follows the contact member 88 until a nut 88 threaded to the former engages the dome 88. This engagement takes place just prior to the time that the ledge 18 engages the lower wall 88 of the housing 84 so that in this embodiment a small portion of the dielectric fluid entrapped in the pressure zone 68 may continue to flow downward immediately around the contact member 28 or 28 through the opening 81. A

similar construction can be used in the form of the invention shown in Figs. 1 to 5.

The passage 88' shown in Fig. 6 is external of the first contact member 28, for example. Referring particularly to the interrupting set of contact members 28, the passage 88' is formed through the wall 88 and the material of the tube ll, being preferably inclined so as to be directed into a space 88 formed by the member 48. The position and inclination of this passage 88' are such that it discharges adjacent the gap between the contact members 28 and 29, and, further, it is preferred that the direction of discharge be toward the corresponding outer end of the tube II, as best shown in Fig. 6. Likewise, the passage 88' of the other interrupting set 24 is inclined to discharge toward the other end of the tube II. The pump means associated with the generating set of contact members 28 preferably aflords a plurality of passages 88' inclined to intersect at or near the axis of; movement of the contact member 88. On the other hand, only one such passage can be used in some instances, or an internal passage such as shown in Fig. i can be employed in this connection. Likewise, it will be understood that both internal and external passages on any or all of the sets 28, 24, or 28 can be employed withoutdeparting from the spirit of this invention.

The invention is not limited to the use of a plurality of sets of contact members even though it has been thus disclosed. 'Advantageous results accrue from the use of a pump means on a singie set of contact members. On the other hand, in interrupting high voltage circuits, it is often desirable to use a plurality of sets, and in some instances to employ a plurality of arc-quenching units of the type shown in Fig. 1. Fig. '1 illustrates two such arc-quenching units enclosed in a tank I88 and indicated respectively by the numerals i8 and I8". A high voltage bushing l8l extends downward in the tank and supports the arc-quenching unit M. This supp rt may comprise a member I82 having arms I88 extending outward to support tubes I84 formed of insulating material. These tubes are connected to shields WE? similar to those indicated by the nu-= moral 56 in Fig. 1 but being suitably secured the tube H. ducting rod 6%, best shown in Fig. ii, which in turn is connected by a conductor till, similar to the conductor iii of Fig. i, to one oi? the contact members, for example, the contact 'illelld her iii.

The other arc-nuenching unit it" is similarly supported irom a bushing tut which, being diagonally disclosed. with reference to the bushing lot, is only partially shown in Fig. 'l. The con ductor We inthis instance is electrically connected to that one of the contact members which is diagonally opposite the contact member to which the conductor M6 is connected.

The six contact members are moved simultaneously by a cross-arm construction 6 it to which they are connected, this cross-arm being moved up and down by any suitable means, such as a crank H6 pivoted to the tank at it?! and con,

nected to any suitable tripping and resetting mechanism disposed outside. the tanlr.

In Fig. 9 the six lower contact members are indicated by the numerals 52b to respectively for the purpose of tracing the circuit. Current may move down through the conductor MS of the bushing it], flowing downward through the first set of interrupting contacts, thence from the contact member E26 through a conductor 9% (corresponding to the conductor 53 of Fig. l), to the contact member Hi, and thence upward through the generating set of contacts, thence through a conductor (not shown in Fig. 9) corresponding to the conductor 5d of Fig. l, the current then flowing downward through the remaining interrupting set to the contact member I22. An electrically conducting arm E29 forming a part of the cross-arm structure H5 carries the current diagonally across the tank to the contact member I23, whence it flows upward, across and downward to the contact member are, thence through conductor I30 to the contact member I25, and thence upward through the corresponding upper contact member which is connected to the conductor I06 of the bushing Hill. The remaining arms 132 and I33 of the cross-arm structure H5 are formed of wood or other insulating material so that current is not conducted thereby. This manner of conducting the current in series through a plurality of sets. of contacts has been found very advantageous 'in circuit breaker design.

While the invention has been described with reference to two preferred embodiments and with reference to a tank-type circuit breaker, it will beclear that the invention is not limited thereby and finds applications in various types of circuit breakers.

I claim as my invention:

1. In a high-voltage circuit breaker adapted to interrupt a circuit by forming and extinguishing an arc in a zone containing a dielectric fluid, the combination of a first contact member electrically connected to said circuit; a second contact member electrically connected to said circuit and normally engaging said first contact mem= her to close said circuit; means for movably mounting said second contact member for movement away from said first contact member to form a gap therebetween; means for movably mounting said first contact member for limited movement while in engagement with said second contact member and before separation thereirom; pump means comprising stationary and The bushing ici carries a con a see out i iii movable elements cooperating in defining a pressure zone containing a portion oi said dielectric fluid; means for operatively connecting movable element and said first contact member to build up a pressure in said pressure acne dun ing said movement; and walls donning a passage communicating with said pressure one and discharging at a position adjacent said gap for conducting dielectric fluid tosaid' 2. In a high-voltage circuit breaker adapted to interrupt a circuit by forming and extinguislnng an arc in a zone containing a dielectric fluid, the combination of: pump means including stationarr and movable elements cooperating in defining a pressure zone containing a dielectric fluid; a first contact member connected to said circuit and operatively associated with said movable element to move same when said first contact memher is moved; a second contact member con nected to said circuit; actuating means for mov= ing said second contact member into engagement with said first contact member and thus into operative relation with said movable element of said pump means and. for moving said second contact member entirely away from said first contact member to form a gap therebetween; and walls defining a passage communicating with said pressure zone and discharging dielectric fluid therefrom at a position adjacent said gap.

3. In a high-voltage circuit breaker adapted to interrupt a circuit by forming and extinguishing an arc in a zone containing a dielectric fluid, the combination of: pump means including stationary and movable elements cooperating in defining a pressure zone; means for biasing said movable element of said pump means in a direction tending to decrease the volume of said pressure zone; a first movable contact member connected to said circuit for retracting said movable element against the action of said biasing means to increase the volume of said pressure zone; intalre means for delivering dielectric fluid to said pressure zone when the volume of said pressure zone increases; a restricted discharge means for discharging dielectric fluid from said pressure zone when the volume thereof decreases; a second contact member connected to said circuit; and means for moving said second contact memher into and from engagement with. said first contact member, said movement of said second contact member being sumcient to move said first contact member enough to retract said movable element of said pump means.

4-. In a high-voltage circuit breaker adapted to interrupt a circuit by forming and extinguishing an arc in a zone containing a dielectric fluid, the combination of: pump means including stationary and movable elements cooperating in defining a pressure zone containing dielectric fluid; first and second movable contact members respectively connected to said circuit and engaging to complete said circuit; means for moving said first and second contact members together and in engagement with each other for a fraction of the locus of motion of said second contact member and for then retarding the motion of said first contact member whereby continued movement of said second contact member forms a gap between said contact members; walls defining a passage communicating with said pressure zone and discharging adjacent said gap; and means for maintaining a flow of dielectric fluid from said pressure zone along and from said passage substantially throughout the time'that said secand contact member is moving to increase the distance across said gap.

5. In a high-voltage circuit breaker adapted to interrupt a circuit by forming and extinguishing an arc in a zone containing a dielectric fluid, the combination of: a pair 01 members defining a longitudinal passage containing dielectric fluid and providing aligned openings; a first contact member of a size to be extensible through one of said openings; means for movably mounting said first contact member in alignment with said one of said openings to move toward and away from this opening, said means including means for preventing any movement of said first contact member entirely through said longitudinal passage; a second contact member extensible through said openings to engage said first contact member and retractibie entirely from said longitudinal passage whereby said contact members separate in said passage to form a gap; pump means including stationary and movable elements cooperating in defining a pressure zone containing dielectric fluid; and means for delivering a stream of dielectric fiuid from said pressure zone to said gap when said second contact member is retracted from engagement with said first contact member, said means including walls defining a passage communicating with said pressure zone and discharging dielectric fiuid therefrom into said longitudinal passage at a position adjacent said gap.

6. In a high-voltage circuit breaker adapted to interrupt a circuit, the combination of a pump means including a housing providing a cylinder and a piston in said cylinder defining a pressure zone which varies in volume when said piston is moved; spring means for biasing said piston in a direction to reduce the volume or said pressure zone; intake means for delivering dielectric fluid to said pressure zone when the volume thereof increases; means ior moving said piston, said means including a movable first contact member; a second contact member movable into engagement with said first contact member to move same to compress said spring and movable away from said first contact member to form a gap; and walls defining a passage communicating with said pressure zone for discharging a stream of dielectric fiuid' at a position adjacent said gap, said stream being impelled by said spring means.

7. A combination as defined in claim 6 including an auxiliary spring disposed operatively between said first contact'member and said piston to be compressed therebetween, whereby said auxiliary spring tends to force said first contact member toward said second contact member, and including means for limiting the extent of movement or said first contact member to permit said second contact member to separate therefrom to form said gap.

8. A combination as defined in claim 6 in which said piston provides an opening and in which said first contact member slidably extends through said opening, and including interengaging means acting between said piston and said first contact member to move said piston along with said first contact member to compress said spring means, said interengaging means permitting iree movement of said first contact member in an opposite direction.

9. A combination as defined in claim 6 in which said piston provides an opening and in which said first 'contact member slidably extends through said opening, and including interengaging means acting between said piston and said first contact member to move said piston along with said first contact member to compress said spring means, said interengaging means permitting free move- -ment or said first contact member in an opposite direction, and including an auxiliary spring acting between said first contact member and said piston be compressed prior to the time said interengaging means acts to move said piston along with said first contact member.

l0..In a high-voltage circuit breaker adapted to interrupt a circuit, the combination of: two contact members connected to said circuit and separable to form a gap when it is desired to interrupt said circuit; a pump mean providing a pressure zone around one of said contact members and containin a dielectric fiuid: and walls defining a passage extending outward at one end to communi ate with said pressure zone and extending in' mally 01' said one or said contact members to openly communicate with said gap to force dielectric fiuid thereinto when said gap i formed.

11. In a high-voltage circuit breaker adapted to interrupt a circuit, the combination of: two contact members connected to said circuit and separable to form a gap when it is desired to interrupt said circuit; a pump means providing a pressure zone containing a dielectric fiuid; and walls defining a passage communicating at one end with said pressure zone and extending internally of one of said contact members to communicate openly with said gap to force dielectric fluid thereinto when said gap is formed, said internally-passaged contact member and said other of said contact members providing contact surfaces engaging each other in sealing relationship until said gap is formed to prevent substantially any or said dielectric fiuid from escaping through said passage until said contact members separate to form said gap.

12. In a high-voltage circuit breaker adapted to interrupt a circuit, the combination of: pump means including movable and stationary elements cooperating in defining a pressure zone containing a dielectric fluid, said stationary element providing an opening; spring means for biasing said movable element in a direction to decrease the volume of said pressure zone; a first contact member movably extending through said opening into said pressure zone to actuate said movable element in a manner to compress said spring means, there being a small space between said first contact member and the walls oi said opening; a second contact member movable into engagement with said first contact member to move same to compress said spring; and stop means for limiting the extent of movement of said first contact member while in engagement with said second contact member to permit said second contact member to move away from said first contact member to form a gap.

13. A combination as defined in claim 12 in which said stop means include a shoulder on said second contact member movable into engagement with said stationary element and extending outward to seal said small space against discharge of dielectric fiuid thereihrough.

14. In a high-voltage circuit breaker adapted to interrupt a circuit, the combination of: two contact members connected to said circuit and separable to form a gap when it is desired to interrupt said circuit, one of said contact members providing a passage with an exit portion opening on said gap and providing an annular contacting surface around said exit portion of said passage, the other of said contact members being imperforate to provide a contacting surface extending across said exit portion of said passage and also across said annular contacting surface whereby when said contacting surfaces are in engagement said passage is effectively sealed; and means for delivering a dielectric fluid under pressure to said passage at an instant of time prior to the separation of said contacting surfaces to form said gap and continuing until said gap is formed, the engagement between said contacting surfaces substantially sealing said passage to prevent escape of dielectric fluid therefrom until said gap is formed, said means acting upon formation of said gap to permit flow of said dielectric fluid under pressure along said passage and toward said imperforate contacting surface to spread said dielectric fluid outward to sweep across said annular contacting surface and thus across the arc terminating at some portion of thisannular surface.

15. A combination as defined in claim 14, in which said means includes a variable-volume chamber containing a dielectric fluid, and inciudes means for decreasing the volume of said chamber to build up a pressure therein prior to the separation of said contacting surfaces and for continuing to decrease the volume of said chamber after said gap is'fcrmed to force a stream of said dielectric fluid through and along said passage to contact saidarc.

ERNEST K, SADLER, 

